Find a combinatorial proof of the above identity, where n and a are positive integers.

I'm not sure how to prove this identity. I know the LHS is the number of multisets of size n with a types and the RHS is the sum of products of numbers of (n-2k) subsets of an a-element set and k-multisets of a types.

+ - 1 - 1 + - 1 - 1